Fretting and whisker resistant coating system and method

ABSTRACT

An electrically conductive material coated with a plurality of layers, includes a metal or metal alloy substrate; a barrier layer deposited on said substrate effective to inhibit diffusion of constituents of said substrate to said plurality of layers; a sacrificial layer deposited on said barrier layer effective to form intermetallic compounds with tin; a low resistivity oxide metal layer deposited on said sacrificial layer; and an outermost layer of tin or a tin-base alloy directly deposited on said low resistivity oxide metal layer.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a continuation of and claims priority to U.S. PatentApplication

No. 10/962,917, filed Oct. 12, 2004, which is now U.S. Pat. No.7,391,116, issued Jun. 24, 2008, which claims priority to U.S.Provisional Patent Application Ser. No. 60/511,249 that was filed onOct. 14, 2003. The subject matter of that provisional patent applicationis incorporated by reference in its entirety herein.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to systems and methods for coating electricallyconductive substrates, and more particularly to multi-layer systems andmethods for coating electrically conductive substrates.

(2) Description of the Related Art

Throughout this patent application, the word “base” to quality an alloymeans that the alloy contains at least 50%, by weight, of the qualifiedelement, for example, “copper-base” means more than 50%, by weight, ofcopper. Copper and copper-base alloys (hereinafter generally referred toas “copper”) are commonly used in electrical and electronic industriesfor connectors, electrical harnesses, printed circuit boards, ball gridarrays, leadframes, multichip modules, and the like. While. copperprovides excellent electrical conductivity, it is known to easilyoxidize and tarnish when exposed to elevated temperatures, moisture, orchemicals. Oxidation and tarnishing of copper generally result in highelectrical contact resistance, thereby reducing performance ofelectrical devices. In addition, oxidation and tarnishing of copperreduce the wettability of solder, and generally makes solderingproblematic.

One method to reduce oxidation and tarnishing of copper is to apply atin or tin-base alloy coating (hereinafter generally referred to as“tin”) onto a copper substrate. The tin coating acts as a barrier toprevent or reduce oxidation, and thereby maintain the electricalperformance of the substrate. However, there are many problemsassociated with using tin as a coating layer on a conductive substrate.Rapidly at elevated temperatures, and more gradually at room temperature(nominally 25° C.), the tin coating interdiffuses with the coppersubstrate to form copper-tin intermetallic compounds (IMC's). TheseIMC's reduce the thickness of the tin coating layer and cause anincrease in contact resistance and degradation of solderability.

Exemplary thermal excursions include 250° C. for a few seconds duringwire bonding or encapsulation in a polymer, 300° C. for a few secondsduring reflow and 150° C. for from 8 to 168 hours for a controlledreduction of tin thickness to reduce friction.

One approach taken to reduce the effect of IMC formation and maintain alow contact resistance is to use a thicker tin coating; however, thisapproach not only increases the cost of the parts but also causes somefunctional problems. Where the tin coating is used on an electricalconnector, a thicker coating of soft tin increases friction leading toan increased insertion force, making plugging and unplugging theconnector physically difficult. For electronic devices, a thicker tin ortin alloy coating is also undesirable since the trend is to makeelectronic devices thinner and smaller. Moreover, where the tin coatingis used on leads of an electronic device, a thick tin coating can causeproblems in the coplanarity and fine line definition of the leads.

Another approach taken to reduce the effect of IMC formation is to use atransition barrier layer between the copper substrate and the tincoating to inhibit the growth of IMC. For example, U.S. Pat. No.4,441,118 reports low IMC growth rates using a copper-nickel alloysubstrate with 15-30% nickel.

In another example, a publication by P. J. Kay and C. A. Mackay, inTransactions of the Institute of Metal Finishing, Volume 51, 1979, atpage 169, discusses the use of various metals as transition barrierlayers. In one example, this publication describes a silver barrierlayer having a thickness of 1 micrometer. However, this example wasshown to be undesirable because the silver transition barrier layerresulted in no substantial reduction in the diffusion rate betweencopper and tin. U.S. Pat. No. 4,756,467 to Schatzberg discloses asolderable connector having a copper substrate, a thin layer of silver,a silver-tin alloy layer and an outermost tin layer. The silver-tinalloy layer is formed by a diffusion anneal. Japanese Patent Number2670348 (publication number 02-301573) to Furukawa Electric Co. Ltd.discloses a copper substrate coated with a barrier layer that is nickelor cobalt, followed by silver layer followed by a melt-solidified layerof tin or tin alloy.

Commonly owned U.S. patent application Ser. No. 10/930,316, that wasfiled on Aug. 31, 2004 as a continuation of U.S. patent application Ser.No. 09/657,794 discloses a thin anti-tarnish layer disposed between acopper substrate and a tin coating layer. Among the metals disclosed asanti-tarnish layers are zinc, chromium, indium, phosphorous, manganese,boron, thallium, calcium, silver, gold, platinum, palladium andcombinations and alloys thereof.

Other barrier layers are disclosed in commonly owned U.S. Pat. No.5,780,172, to Fister et al., and commonly owned U.S. Pat, No. 5,916,695to Fister et al. U.S. Pat. Nos. 4,756,467 and 5,916,695 as well as U.S.patent application Ser. No. 10/930,316 are incorporated by reference intheir entireties herein

Another problem associated with the use of tin as a coating layer for aconductive substrate is that tin is susceptible to fretting corrosion.Fretting corrosion is the oxidation of contact surfaces that resultsfrom relative motion (fretting) between two mating contact surfaces. Theoxidation caused by fretting can result in an unacceptable increase incontact resistance. Certain metals, such as silver, are known to haveexcellent resistance to fretting corrosion. However, silver tends totarnish in the atmosphere due to the presence of sulfur dioxide, whichcauses silver sulfide to form on the surface of the silver. The tarnishis aesthetically unacceptable and could degrade the functionalproperties of the electrical contact.

Yet another problem associated with the use of a tin coating layer, aswell as other coating layers such as zinc, indium, antimony, or cadmium,on a conductive substrate is that tin is susceptible to whiskering.Whiskering occurs as the tin ages and stresses in the tin or at thetin/IMC interface begin to build. Whiskering also occurs due to internalstress resulting from the plating process. To relax the stress, singlecrystals of tin nucleate from the surfaces like whiskers. Each whiskercontinues to grow until the internal stresses subside. Whiskering cancause many different problems, including shorting of adjacent electricalcontact surfaces. Alloying the tin coating with a small amount of lead(Pb) is a common means of reducing whisker growth, However, because ofhealth and environmental reasons, many industries are striving to reduceor eliminate the use of lead.

Thus, there is a need to develop a coating system that would be able tomaintain a low contact resistance and good solderability after frettingand thermal exposure, combined with one or all of the additionalattributes of lower coefficient of friction, and reduced whisker growth.

BRIEF SUMMARY OF THE INVENTION

In accordance with a first embodiment of the invention, there isprovided a coated electrically conductive substrate having particularutility where there are multiple closely spaced features and a tinwhisker constitutes a potential short circuit. Such substrates includeleadframes, terminal pins and circuit traces such as on printed circuitboards and flexible circuits and the features include leads, lines andcircuit traces. The electrically conductive substrate has a plurality ofleads separated by a distance capable of bridging by a tin whisker, asilver or silver-base alloy layer coating at least one surface of atleast one of the plurality of leads, and a fine grain tin or tin-basealloy layer directly coating said silver layer.

In accordance with a second embodiment of the invention, there isprovided a coated electrically conductive substrate having particularutility where the debris from fretting wear may oxidize and increaseelectrical resistivity, such as in a connector assembly. Theelectrically conductive substrate has a barrier layer deposited on thesubstrate that is effective to inhibit diffusion of constituents of thesubstrate into a plurality of subsequently deposited layers. Thesubsequently deposited layers include a sacrificial layer deposited onthe barrier layer that is effective to from intermetallic compounds withtin, a metal that is capable of forming a low resistivity oxide(referred to herein as a “low resistivity oxide metal layer”) depositedon said sacrificial layer, and an outermost layer of tin or a tin-basealloy directly deposited on the low resistivity oxide metal layer

In this second embodiment, the barrier layer is preferably nickel or anickel-base alloy and the low resisitivity oxide metal layer ispreferably silver or a silver-base alloy.

When heated, the coated substrate of this second embodiment forms aunique structure having a copper or copper-base alloy substrate, anintervening layer formed from a mixture or metals including copper andtin, and an outermost layer which is a mixture of a copper-tinintermetallic containing phase and a silver-rich phase.

It is believed that this silver-rich phase is particularly beneficial tominimize an increase in resistivity due to oxidation of fretting weardebris.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following detaileddescription taken in conjunction with the accompanying drawings whereinlike elements are numbered alike, and in which:

FIG. 1 is a top planar view of a leadframe prior to encapsulation andcoating in accordance with a first embodiment of the invention;

FIG. 2 is side planar view of the leadframe of FIG. 1 subsequent toencapsulation but prior to coating in accordance with the firstembodiment of the invention;

FIG. 3 is a cross-sectional view of the leadframe of FIG. 1 subsequentto encapsulation and coating in accordance with the first embodiment ofthe invention.

FIG. 4 is a cross-sectional view of a conductive strip coated inaccordance with a second embodiment of the invention.

FIG. 5 is a cross-sectional view of the conductive strip of FIG. 4subsequent to being formed into a connector assembly.

FIG. 6 is a magnified cross-sectional view of a portion of the connectorassembly of FIG. 5 illustrating the effect of fretting debris.

FIG. 7 is a flow chart representation of a process to manufacture thefirst embodiment of the invention,

FIG. 8 is a flow chart representation of a process to manufacture thesecond embodiment of the invention.

FIG. 9 illustrates the interdiffusion between layers in coatedsubstrates with different layer combinations.

FIG. 10 is a photomicrograph of a surface of a coated substrate of theinvention following heating to 150° C. for one week.

FIG. 11 is a photomicrograph of a cross-section of the coated substrateof FIG. 10.

DETAILED DESCRIPTION

Referring to FIG. l, a leadframe has a plurality of leads 10 are formedfrom an electrically conductive metal, such as copper or a copper-basealloy. Each of the plurality of leads 10 terminates at an inner lead end12 to define a central aperture that is occupied by a die paddle 14.Typically, the inner lead ends 12 and die paddle 14 are coated with athin layer of a precious metal, such as silver, to enhance chip attachand wire bonding. When silver, this thin layer has a typical thicknessof between 3 microns and 6 microns and is deposited byelectrodeposition. One or more integrated circuit (IC) devices 16,commonly referred to as semiconductor chips, are then attached to thedie paddle 14, such as through the use of a low temperature metallicsolder or a thermally conductive polymer adhesive. Thin metal wires 18,or thin strips of conductive metal foil, electrically interconnectcircuits on an electrically active face of the integrated circuit device16 to the inner lead ends 12. A molding resin then encapsulates the diepaddle 14, integrated circuit device 16, inner lead ends 12 and leadmid-portions 21, generally along the perimeter identified by the brokenline 20.

FIG. 2 is a side planar view of the assembly showing leads 10 extendingfrom the molding resin 22. Outer portions 23 of the leads extending fromthe molding resin are typically soldered to external circuitry, such astraces on a printed circuit board. To maximize electrical conductivity,the leads are typically formed from copper or a copper alloy, althoughnon-copper leads such as iron-nickel and iron-nickel-cobalt alloys areused. Copper and copper alloys readily oxidize and the formation of anoxide on the surface. impedes soldering.

To inhibit oxide formation, it is common to deposit a tarnish resistantlayer on the copper leads. One readily solderable material for theanti-tarnish layer is tin or a tin-base alloy. When exposed totemperatures at or above room temperature, there is diffusion betweenthe copper and the tin. As a copper-tin intermetallic forms on thesurface of the layer, the anti-tarnish characteristics and solderabilityboth degrade. It is known to dispose a barrier layer, such as nickel,between the anti-tarnish layer and the substrate to reduce the rate ofdiffusion and reduce the rate of intermetallic formation.

Tin whiskering refers to a characteristic of tin where internal stressesare relieved by the growth of thin tin filaments. Referring back to FIG.1, the leads 10 are closely spaced and it is possible for a tin whiskerto bridge the gap 24 between adjacent leads creating an electrical shortcircuit. Typically, a gap distance between leads of 1 millimeter or lessis at risk of being bridged by a tin whisker. While numerous solutionsfor preventing tin whiskering have been proposed, these solutions havelimitations. It is known reduce whiskering by alloying the tin withanother metal, such as lead, but lead is toxic. It is known to heat thetin to above its melting temperature, a process known as reflow, toreduce whiskering. It is difficult to control the flow of molten tin andbridging between leads frequently results during reflow.

In accordance with a first embodiment of the invention and referring toFIG. 3, tin whiskering is reduced by forming the lead 10 from asubstrate 26 that is coated with a layer of silver or a silver-basealloy 28, followed by a layer of fine grain tin 30 directly deposited onthe layer of silver or silver-base alloy. By “directly” deposited, it ismeant deposited to be adjoining, without any intervening layers of othermaterials. If the substrate 26 is formed from a metal other than copperor a copper base alloy, a thin, on the order of 1-20 microinches layerof copper may be deposited on the substrate prior to deposition of thelayer of silver 28. The layer of silver 28 may be a silver-base alloyand the layer of tin may be a tin-base alloy.

The interface between two metals tends to have less strength than themetal itself. Therefore, it is preferred that the portion of the leads10 encapsulated by molding resin are not coated with the layers ofsilver and tin and these layers coat only those portions of the leadswhich extend outward from the molding resin, The layer of silver 28 hasa thickness of between 1 microinch and 120 microinches. When thethickness is below 1 microinch, tin whiskering is not adequatelysuppressed. When the thickness is about 120 microinches, the costbecomes prohibitive. A preferred silver thickness is from 2 microinchesto 40 microinches and a most preferred silver thickness is from 5microinches to 20 microinches.

The layer of tin 30 has a thickness of between 0.01 microinch and 400microinches. When the tin thickness is less than 0.01 microinch, tarnishresistance and solderability are degraded. When the tin thicknessexceeds 400 microinches, bridging between adjacent leads is likely. Apreferred tin thickness is from 20 microinches to 150 microinches and amost preferred tin thickness is from 20 microinches to 80 microinches.

The tin is fine grain, as achieved by electrodeposition, as opposed tocoarser grain as achieved following reflow. Typically, the average grainsize is from 0.1 micron to 100 microns, and preferably from 0.5 micronto 5 microns, as opposed to after reflow when the nominal grain size ison the order of millimeters. The fine grain structure has a generallyhigher ductility enabling the leads to be bent to a sharper radiuswithout fracture of the coating layer. While a fine grain tin isbelieved more prone to whisker formation, the underlayer of silverenables use of fine grain tin in this embodiment.

While this first embodiment has been described in view of a leadframehaving closely spaced leads, other structures such as terminal pins,printed circuit boards and flex circuits having other closely spacedfeatures such as lines and circuit traces also benefit from thewhisker-free coatings of the invention.

A second embodiment of the invention is drawn to connector assemblies.Unlike leadframes, most connector assemblies are not affected by tinwhiskers since adjacent connectors are usually spaced apart far enoughto avoid short circuiting by tin whiskers. Also, since connectors arenot closely pitched like leads of a leadframe, reflow is available toreduce internal stresses in a tin coating. Further, diffusion betweentin and copper is frequently desirable to reduce the thickness of freetin, thereby reducing friction and reducing the force needed to insert aprobe into a socket.

Connector assemblies are subject to an increase in resisitivy due tofretting debris. Fretting wear is a phenomenon that occurs between twosurfaces having oscillatory relative motion of small amplitude. Frettingwear causes the removal of small particles from the contacting surfaces.These small particles subsequently oxidize and the oxidized debrisaccumulates at the interfaces of the connector assembly. Since the roomtemperature resistivity of tin is about 0.12 μΩ·m while the roomtemperature resistivity of tin oxide is about 1 μΩ·m, the result offretting wear is a degrading of the connector assembly electricalproperties.

Fretting wear is reduced by forming the connector assembly in accordancewith a second embodiment of the invention. With reference to FIG. 4, asubstrate 26 is typically copper or a copper-base alloy, although otherelectrically conductive metals may be used. When one of those otherelectrically conductive metals is employed, a thin copper layer isdeposited on the substrate as described above. The thin copper layer mayalso be deposited on a copper-base alloy substrate to provide a purecopper surface to facilitate deposition and adherence of subsequentlayers.

Deposited on to the copper or copper-base alloy substrate or thin copperlayer is a barrier layer 32. The barrier layer may be any metal thatinhibits the diffusion of copper and the other constituents making upthe substrate and is preferably a transition metal, such as nickel,cobalt, iron, manganese, chromium, molybdenum or their alloys. Thebarrier layer has a thickness of between 2 microinches and 80microinches. If the barrier layer has a thickness of less than 2microinches, it may not be effective to inhibit diffusion. If thebarrier layer thickness exceeds 80 microinches, it may adversely affectthe electrical and mechanical properties of the connector assembly.Preferably, the thickness is between 4 microinches and 40 microinches.More preferably, the thickness of the barrier layer is between 4microinches and 20 microinches.

Deposited on the barrier layer 32 is a sacrificial layer 34, Thesacrificial layer 34 is a metal that combines with both silver and tinto form alloys and intermetallic compounds. To reduce friction, the freetin thickness of outermost layer 36 is reduced. This thickness reductionmay be accomplished by heating the assembly such that the sacrificiallayer combines with the inner portion of the outermost layer to formrelatively hard intermetallic compounds. A preferred material for thesacrificial layer is copper or a copper-base alloy having a thickness ofbetween 2 microinches and 60 microinches. The thickness of thesacrificial layer is selected such that when the sacrificial layer isconsumed, at least a thin, on the order of 2 microinches, layer of freetin preferably remains on the exterior surface 38 of the outermost layer36. When the outermost layer is initially between 40 microinches and 80microinches of tin, a copper sacrificial layer has a most preferredthickness of between 5 and 20 microinches.

Disposed between the sacrificial layer 34 and the outermost layer 36 isa low resistivity oxide metal layer 40. This low resistivity oxide metaloxide is a metal that forms an oxide at the anticipated operatingtemperature of the connector assembly that has a resistivity which isless than the resistivity of tin oxide, Silver or a silver base alloy ispreferred for the low resistivity oxide metal layer 40. While tin oxidehas a room 30 temperature resistivity of about 1 μΩ·m, silver oxide hasa room temperature resistivity of about 0.14 μΩ·m. By including silveroxide as a component of the fretting debris, the impact of fretting wearon the resistivity of the connector assembly is significantly reduced.The low resistivity oxide metal layer has a thickness of between 1microinch and 120 microinches. If the thickness is less than 1microinch, there is insufficient silver oxide to affect the connectorassembly resistivity. If the thickness exceeds 120 microinches, costbecomes prohibitive. Preferably, the thickness of the low resistivityoxide metal layer is between 2 microinches and 40 microinches and mostpreferably, between 5 microinches and 20 microinches.

The conductive strip of FIG. 4 is formed into a connector assembly asshown in cross-sectional representation in FIG. 5. The connectorassembly includes a socket 42 and probe 44. The socket is usually bentinto a shape effective to make point contact with the probe with theshape also imparting an internal stress in the socket effective to applya positive force to maintain electrical contact at the point 46.

FIG. 6 is a magnified view of the point contact identified by a brokencircle in FIG. 5. Due to vibration, the point 46 oscillates between afirst contact point 48 and a second contact point 50. This frettinggenerates fretting debris 52 in the form of metal oxides. A portion 54of the fretting debris coats the oscillation track and impacts the flowof electric current between the point 46 and probe 44.

The low resistivity oxide metal layer is any metal that both forms anoxide with a resistivity less that of tin oxide (1 μΩ·m) or is a metalmore noble than silver that has a low tendency to form oxides, such asgold, platinum and palladium. Table 1 identifies the oxides of many basemetals and reports their suitability for use as the low resistivityoxide metal layer. In Table 1, “O” indicates suitability and “X”indicates not suitable. As alternatives to silver, indium, iron,niobium, rhenium, ruthenium, vanadium, gold, platinum, palladium andzinc, as well as mixtures of these four metals are suitable.

TABLE 1 Room Temperature Resistivity Base Metal Oxide (Ω · m)Suitability Resistivity Cadmium CdO 3.90E−05 X - Toxic Acceptable IndiumIn₂O₃ 1.00E−03 ◯ Iron Fe₃O₄ 1.00E−04 ◯ Niobium Nb₂O₃ 8.60E−04 ◯ RheniumReO₃ 2.00E−05 ◯ Ruthenium Ru₂O 3.52E−07 ◯ Silver AgO 0.14 ◯ Uranium UO₂3.80E−02 X - Toxic Vanadium V₂O₃ 5.50E−05 ◯ Zinc ZnO 1.50E−02 ◯Resistivity Bismuth Bi₂O₃ 1.00E+07 Not Cobalt CoO 1.00E+06 AcceptableCopper Cu₂O 2.00E+05 Europium EuO 1.00E+06 Manganese MnO 1.00E+06 NickelNiO 1.00E+11 Silicon SiO₂ 1.00E+12 Sodium Na₂0₂ 2.50E+02 Tantalum Ta₂O₅1.00E+03

FIG. 7 is a flow chart representation of a method for the production ofthe coated substrate illustrated in FIG. 3 for use in applications wheretin whiskering is a concern and tin reflow is not an option to relieveinternal stress. Such applications include leadframes, closely spacedterminal pins (such as found in a pin grid array electronic package) andclosely spaced circuit traces on a printed circuit board or flexiblecircuit. Referring back to FIG. 7, the first three process steps arespecific to a leadframe and in some embodiments terminal pins. Theremaining three steps are generic to all the above product lines.

A leadframe is either stamped or chemically etched from a substrate,typically copper or a copper base alloy. The leadframe includes acentrally disposed die paddle and a plurality of leads extendingoutwardly from at least one, and typically all four sides of the diepaddle. The leadframe is then degreased and cleaned, such as by acommercial degreaser, for an alkaline electrocleaner such asHubbard-Hall E-9354 electrocleaner (available commercially fromHubbard-Hall, Waterbury, Conn.). An alkaline mixture along withanodic/cathodic electrocleaning allows generated oxygen or hydrogenbubbles to remove most impurities residing on the substrate.Electrocleaning is typically performed at about 20° C. to 55° C. forabout one minute with a current density range of about 10 to 50 asf(amps per square foot).

The die paddle and inner portions of the leads are then coated 56 with ametal that enhances solderability and wire bonding, such as silver to athickness of from 3 microns to 6 microns. It is preferable that only theinnermost portion of the leads, the part utilized for wire bonding ortape automated bonding (TAB) be silver coated. This is because duringthe subsequent encapsulation step 58, it is desired for the moldingresin to directly contact the copper substrate, providing a singleinterface for adhesion failure and moisture egress. Less preferred is atwo interface arrangement where the molding resin contacts the silverlayer that contacts the copper substrate. The silver coating 56 may beby any suitable process, such as electrodeposition, electrolessdeposition, immersion coating, chemical vapor deposition or plasmadeposition.

The IC device is then bonded to the die paddle by convention die attachmethods 60 such as soldering with a low temperature solder, for examplea gold/tin eutectic, or adhesive joining, such as with a metal filledepoxy. Wire bonding employs small diameter wires or thin strips of metalfoil to electrically interconnect the IC device to the inner leadportions of the leadframe. Subsequent to die attach and wire bonding,the die paddle, IC device, wire bonds and inner lead portions of theleadframe are encapsulated in a thermosetting molding resin, such as anepoxy. The outer portions of the leads are then bent into a desiredshape for bonding to a printed circuit board or other external circuit.

The outer lead portions are then coated 62 with a layer of silver orsilver alloy, by any suitable process, such as electroplating,electroless plating, immersion plating, physical vapor deposition,chemical vapor deposition, plasma deposition or metal spraying. Thesilver is applied to a thickness of from 1 microinch to 120 microinches,with a thickness of from 2 microinches to 20 microinches being mostpreferred.

A preferred method of depositing the layer of silver is byelectroplating from an aqueous solution containing 31-56 grams per litersilver cyanide, 50-78 g/l potassium cyanide, 15-90 g/l potassiumcarbonate and brighteners. Electroplating is at a temperature of between20° and 28° C. at a current density of between 5 amps per square footand 15 asft Alternatively, the silver layer can be deposited usingcyanide-free immersion plating such as MacDermid Sterling™ silver(MacDermid Inc., Waterbury, Conn.).

A layer of tin is then coated 64 on the Ag coated outer leads to athickness of from 0.006 microinch to 400 microinches and preferably to athickness of from 20 microinches to 80 microinches, A preferred methodof depositing the layer of tin is by electroplating from a solutioncontaining methane sulfonic acid based tin plating solution such as Rohmand Haas Solderon™ ST200 (Rohm and Haas Company, Philadelphia, Pa.) orMacDermid StanTek™ AMAT for matte tin. MacDermid StanTek™ Stellite isuseful for bright tin. Typical operating conditions for the aboveelectrolytes are a temperature of 25° C.-35° C. and a current density ofbetween 5 asf and 50 asf.

The tin coated exterior leads are then bonded 66 to a printed circuitboard or other external circuit, such as by soldering using either atin/lead alloy solder or appropriate leadfree solder. The solder andsoldering process are selected to enable the solder to fuse to the layerof tin without the layer of tin melting. Melting of the layer of tin isto be avoided to prevent bridging of liquid solder between leads.

FIG. 8 is a flow chart representation of a method for the production ofthe coated substrate illustrated in FIG. 4 for use in applications wherethe effect of the oxidized debris of fretting wear on electricalresistivity is a concern, such as in electrical connector assemblies.Referring back to FIG. 7, when the substrate is not copper, or where thesubstrate is a copper alloy having a high (for example greater than 2%,by weight) alloy content, it is desirable to deposit 68 a thin copperlayer on the surfaces of the substrate prior to deposition of subsequentlayers. The thin copper layer minimizes the effect of different metalson the deposition of subsequent layers leading to more consistentproduct performance for many different substrate materials.

The copper layer has a minimum thickness of 5 microinches and has atypical thickness of between 20 microinches and 40 microinches. Whilethe copper layer and the subsequent layers described below may bedeposited by any suitable method, a preferred method of depositing 68the layer of copper is by electroplating from an aqueous solutioncontaining from 20 g/l to 70 g/l of copper ions and from 50 g/l to 200g/l of sulfuric acid. Operating conditions are a temperature of from 40°C. to 60° C. at a current density of from 20 asf to 100 asf.

A barrier layer is next deposited 70. Suitable barrier layers includenickel, cobalt, chromium, molybdenum, iron and manganese and theiralloys or mixtures deposited to a thickness of from 2 microinches to 40microinches and preferably to a thickness of from 4 microinches to 20microinches. A preferred method of depositing 70 the layer of nickel isby electroplating from an aqueous solution nominally containing 300 g/lof nickel sulfamate, 6 g/l of nickel chloride and 30 g/l of boric acid.Operating conditions are a temperature of from 28° C. to 60° C., a pH offrom 3.5 to 4.2 and a current density of from 2 asf to 30 asf.

A sacrificial layer, such as of copper, is next deposited 72 to athickness effective to combine with a portion of the tin during acontrolled thermal excursion to form copper/tin intermetallics such asCu₃Sn, Cu₆Sn₅, and (Cu alloy)_(x)Sn_(y), while retaining a layer ofessentially pure tin (referred to as free tin) on the surface. The layerof free tin should be on the order of 2 microinches to 120 microinchesto provide a solderable, tarnish resistant, layer. The intermetalliclayer is useful to reduce friction by reducing the thickness of softfree tin. Reduced friction leads to reduced insertion force required forthe connector assembly.

After depositing 72 the sacrificial layer, a metal that forms a lowresistivity oxide, such as silver, is deposited 74. The sacrificiallayer is deposited to a thickness of from 1 microinch to 120 microinchesand preferably to a thickness of from 5 microinches to 20 microinches. Apreferred method of depositing the sacrificial layer of silver is byelectroplating from an aqueous solution containing silver cyanide or byimmersion plating from a cyanide-free solution as described above. Inaddition to silver, indium, iron, niobium, rhenium, ruthenium, vanadium,gold, platinum, palladium and zinc, as well as mixtures of these metals,may be employed as illustrated above in Table 1.

After depositing 74 the sacrificial layer, an outermost layer of a metalhaving a melting temperature less than the melting temperature of anyone of the substrate, the barrier layer, the sacrificial layer and thelow resistivity oxide metal layer, is deposited 76. Tin or tin-basealloys are preferred for the outermost layer, For most applications,lead is avoided for toxicity concerns; however, a tin-base alloycontaining lead may be suitable for some applications. The outermostlayer is deposited 76 by any of the methods described above or by tindeposition specific methods such as HALT (hot air level tin) process andmechanical wipe. The outermost layer can have a bright or matte finish,as desired. A matte finish may be produced by electroplating tin from atin bath that is known in the art for preparing this type of finish.Suitable electrolytes include Solderon™ ST200 and StanTek™ AMAT asdescribed above.

The tin is then reflowed 78, such as by heating to a temperature abovethe melting point of tin (232° C.) to reflow the tin. A preferredthermal profile is 300° C. for a few (1-10) seconds in air or in aprotective atmosphere, such as nitrogen. The molten tin is then quenchedto produce a lustrous appearance.

Either before or after reflow the coated substrate is formed 80 into adesired component, such as part of a connector assembly. The coatedsubstrate may also be heated in air or nitrogen at a temperature lessthan the melting temperature of the tin to increase the amount ofintermetallic and reduce the free tin to a desired thickness, typicallyfrom 2 microinches to 20 microinches. This heating may be at atemperature of from 150° C. to 200° C. for from 1 hour to 168 hours.

A mechanism for the improved coatings of the invention may be understoodwith reference to FIGS. 9 a through 9D. FIG. 9A illustrates a tin coatedsubstrate 26 as known form the prior art. The substrate 26 is coatedwith a sacrificial layer of copper 34 and an outermost tin layer 36.After exposure to elevated temperatures, such as 150° C. for one week,interdiffusion and combining occur between the sacrificial layer 34 andoutermost layer 36 to form a Cu₃Sn intermetallic layer 82 adjacent thesubstrate 26 that extends upwards to the surface 84 of the outermostlayer. The outermost layer, after elevated temperature exposure is amixture of the Cu₃Sn intermetallic and the Cu₆Sn₅ intermetallic 86.These two copper base intermetallics are prone to oxidation leading todiscoloration and an increase in resistivity.

FIG. 9B illustrates that, in accordance with the invention, when thesubstrate 26 is coated with a sacrificial layer 34, silver layer 28 andoutermost tin coating layer 36 and then heated to 150° C. for one week,the substrate 26 is coated with an intervening layer 88 that is amixture of copper and tin while the outermost layer is a mixture ofsilver containing Cu₃Sn intermetallic 90 and a silver-rich phase 92. Bysilver-rich, it is meant that the phase contains in excess of 50 atomic% silver. The Cu₃SnAg_(x) intermetallic provides a hard surface toreduce insertion force and reduce fretting wear. The silver-rich phaseprovides tarnish resistance and reduces the increase in resistivity dueto fretting wear debris corrosion.

FIG. 9C illustrates that, in accordance with the invention, when thesubstrate 26 is coated with a barrier layer 32, sacrificial layer 34,silver layer 28 and outermost tin coating 36 and then heated to 150° C.for one week, the substrate 26 is then coated with intervening layer 96that is a mixture of nickel, copper and tin. Adjacent the layer 96 is asecond layer 98 that is a mixture of nickel, copper, silver and tin. Theoutermost layer is a mixture a first component which is Cu₆Sn₅intermetallic and excess tin and a trace of silver and a secondcomponent which is the silver-rich phase 92.

FIG. 10 is a photomicrograph, at a magnification of 2000 times, of theoutermost surface 84 of the coated substrate of FIG. 9C after heating to150° C. for one week. The surface is a mixture of copper-silver-tinphase 98, which appears as dark regions in the photomicrograph, andsilver-rich phase 92, which appears as light regions in thephotomicrograph. FIG. 11 is a photomicrograph, at a magnification of20,000 times, of the coated structure of FIGS. 9C and 10.

FIG. 9D illustrates that when the substrate 26 is coated with a barrierlayer 32, silver layer 28 and outermost coating layer of tin 36 and thenheated to 150° C. for one week, the substrate 26 is then coated with afirst intervening layer 100 that is a mixture of nickel, copper and tinwith a trace of silver. The first intervening layer 100 is coated with asecond layer 102 that is a mixture of copper, nickel, tin and silver.This second layer 102 extends to the surface of the outermost layerwhich is predominantly the silver-rich phase 92.

The advantages of the coating systems of the invention will becomeapparent from the examples that follow. The following examples areintended to illustrate and not to limit the scope of the presentinvention.

EXAMPLES Example 1 Tin Whiskering

Coupons 2 inches×0.5 inch×0.010 inch thick were cut from a strip ofcopper alloy C194. Copper alloy C194 has a composition by weight of2.1%-2.6% iron, 0.05%-0.20% zinc, 0.015%-0.15% phosphorous and thebalance is copper. The coupons were cleaned in a commercial alkalinecleaner at 50° C. using a cathodic current density of 15 asf (amps persquare foot) for 1 minute.

Referring to Table 2, when a nickel layer was deposited, deposition wasby electroplating. The Ni plating solution was an aqueous solutioncontaining from about 60 to 75 g/l (grams per liter) Ni as Ni sulfamate,about 6 to 8 g/l NiCl₂, and about 38 to 53 g/l boric acid at 53° C. witha pH between about 3.5 and 4.2. The Ni plating conditions were 30 asffor about 60 seconds.

When a copper layer was deposited, deposition was by electroplating froman aqueous solution containing about 20 to 70 g/l Cu and about 50 to 200g/l H₂SO₄ at 40-60° C. using a current density of 40 asf for about 40sec.

When a silver layer was deposited, deposition was made in an aqueoussolution containing 31-56 g/l silver cyanide, 50-78 g/l potassiumcyanide, 15-90 g/l potassium carbonate and brighteners. Operatingconditions were a temperature of from 20° C. to 28° C. and a currentdensity of from 5 asf to 15 asf.

Tin was deposited by electroplating from the MacDermid StanTek™ AMATsolution for matte tin deposits and from the MacDermid StanTek™ Stellite100 solution for bright tin deposits. The plating conditions were 30 asffor about 50 to 400 seconds at 25° C. to 40° C.

An accelerated tin whisker test was conducted on the samples by bendingand restraining samples in a circular groove with a radius of 3″. Inthis way a constant bending stress was applied to the tin coating toinduce the formation of whiskers. The compressed (concave) sides ofexemplary and comparative samples were periodically examined under anoptical microscope at 500× to observe the formation of tin whiskers.

TABLE 2 Intervening Number of whiskers Layers, per mm² and length ofThickness in Tin longest whisker in microns Tin microinches thick. in 10days 60 days 120 days 180 days Sample Finish Ni Cu Ag μinches # μm # μm# μm # μm 1 matte -0- 20-40 -0- 250-500 25 19  58 28 138 36 295 53 2matte -0- 20-40 -0-  75-130 33 28 298 175  310 87 496 169  3 matte -0-20-40 5-10  75-130 -0- -0- -0- -0- -0- -0- -0- -0- 4 matte -0- 20-40 -0-40-80 108  36 389 45 361 130  512 45 5 matte -0- 20-40 5-10 40-80 -0--0- -0- -0- -0- -0-  5  5 17 matte 5-20 -0- 5-10 40-80 -0- -0- -0- -0--0- -0-  2  4 19 matte 5-20  7-18 5-10 40-80 -0- -0- -0- -0- -0- -0- -0--0- 20 bright -0- 20-40 -0- 40-80  8  7  33  7  30  9 231 28 21 bright-0- 20-40 5-10 40-80 -0- -0- -0- -0- -0- -0- -0- -0-

As shown from Table 2, the inclusion of a silver layer directlycontacting the tin coating layer substantially eliminated the formationof tin whiskers for both a matte tin outermost coating and a bright tinoutermost coating. For matte tin, compare Sample 2 to Sample 3 andSample 4 to Sample 5. For comparison, Sample 1 is a commercial thick tinproduct. For a bright tin outermost coating, compare Sample 20 to Sample21.

Example 2 Fretting Wear Impact on Contact Resistance

Coupons having the dimensions 6 inches by 1.25 inches by 0.005 inch wereformed from copper alloys C194 and C7025, wrought monolithic tin andwrought monolithic silver as noted in Table 3. C7025 has a composition,by weight, of 2.2%-4.2% nickel, 0.25%-1.2% silicon, 0.05%-0.3% Mg andthe balance is copper.

The copper alloy coupons were coated with intervening layers and mattetin as in Example 1 except that the silver layer was deposited by theimmersion method using MacDermid Sterling™ silver solution and the tinwas deposited from a sulfate solution containing 20 g/l to 80 g/l of tinions as SnSO₄ 50 g/l to 200 g/l sulfuric acid, and organic additives.

The impact of fretting wear on contact resistance was determined bymoving a ¼″ diameter bump at 5 Hz up to 20,000 cycles with a cyclelength of 20 μm across a contact surface to be tested. A normal force of100g was applied to the bump, and contact resistance data was collectedwhile the bump was in motion. The values reported are the number ofcycles required to achieve a specified contact resistance, A highernumber of cycles indicates a better resistance to fretting.

TABLE 3 Surface Cycles to Cycles to Layer 10 mΩ 10 Ω Intervening SurfaceThickness Contact Contact Sample Substrate Layers Layer (μinch)Resistance Resistance 1 C194 Ni/Cu Matte Tin 20 61 3269 2 C194 Ni/Cu/5μinch Ag Matte Tin 20 79 4400 3 C194 None Matte Tin 40 116 2269 4 C194 5μinch Ag Matte Tin 42 490 >5000* 5 Wrought None None N/A 253 6530 Tin 6Wrought None None N/A >20,000 >20,000   Ag *Testing terminated after5000 cycles

Comparing Sample 2 of the invention to Sample 1 demonstrates that theaddition of a 5 μin silver layer was effective to reduce the frettingwear induced resistance of the substrate by about a 30% increase in thenumber of cycles needed to reach 10 mΩ contact resistance and about a35% increase in the number of cycles needed to reach 10 Ω contactresistance.

Comparing Sample 4 of the invention to Sample 3 demonstrates that theaddition of a 5 μin silver layer was effective to reduce the frettingwear induced resistance of the substrate by about a 322% increase in thenumber of cycles needed to reach 10 mΩ contact resistance and in excessof a 120% increase in the number of cycles needed to reach 10 Ω contactresistance.

Monolithic wrought Ag (Sample 6) had better performance than any of thesamples having a coated copper substrate, but is not practical to forman electrical connector due to cost and tarnish. Monolithic wrought Sn(Sample 5) had a reasonably good fretting-resistant probably due to theabundance of free Sn or increased hardness resulting from rolling, butis not practical as a connector due to a lack of strength.

Example 3 Coefficient of Friction

Copper alloy C194 coupons having the dimensions 6 inch×1.25 inch×0.005inch were coated with intervening layers and matte tin as in thepreceding examples. A reflowed tin surface was obtained by heating asample to 350° C. in air and quenching in water.

The coefficient of friction was measured as the ratio of the resistiveforce relative to the normal force (R/N) when a ¼ inch diameter bumpslid at 3 mm/sec for 10 cycles across a tin coated flat surface. Thenormal force was loaded as dead weight and no lubricant was appliedbetween the tin coated surface and the bump. The resistive force wasmeasured as the bump was slid against the flat surface of the sample.The value reported was the average of all 10 cycles. A lower R/Nindicates less friction. The results are reported in Table 4.

TABLE 4 Intervening Layers - Thickness in Tin R/N μinch Thickness 0-60mm Sample Cu Ag Tin Type μinch 100 g 250 g 1 20-40 -0- Matte 40-80 0.550.55 2 20-40 2-5 Matte 40-80 0.58 0.53 3 20-40  5-10 Matte 40-80 0.480.45 4 20-40 10-20 Matte 40-80 0.47 0.46 5 -0- -0- Reflow 40 0.48 0.47 6-0- 5 Reflow 40 0.30 0.22

As R/N decreases, the insertion force required to insert a probe into asocket decreases. Comparing Sample 3 to Samples 1 and 2 shows that anaddition of 5 microns of silver provides about a 14% reduction in R/Nfor a matte tin outer layer. Comparing Sample 3 to Sample 4 shows thatincreasing the silver thickness has no appreciable benefit and leads tohigher costs.

Comparing Samples 5 and 6 shows that a more pronounced benefit, about a45% decrease in R/N, is achieved with reflowed tin as an outer coatinglayer.

Example 4 Interdiffusion Between Layers

Tables 5 through 8 report the measured compositions of the structuresillustrated in FIGS. 9A though 9D to demonstrate the formation of asilver-rich phase on the outermost surface of substrates coated inaccordance with the invention. The thicknesses, in microinches, beforeheating to 150° C. for one week, were measured by XRF (x-rayfluorescence). The composition and atomic percents after heating weredetermined by EDX (energy-dispersed x-ray).

TABLE 5 (FIG. 9A) Ref. No. Ref. No. from FIG. Thickness from FIG. Atomic9A Composition (μinches) 9A Composition Percent 26 C194 N.A. 26 C194N.A. 34 Copper 20-40 82 Copper 75% Tin 25% 36 Tin 40-80 86 Copper 56%Tin 44%

TABLE 6 (FIG. 9B) Ref. No. Ref. No. from FIG. Thickness from FIG. Atomic9B Composition (μinches) 9B Composition Percent 26 C194 N.A. 26 C194N.A. 34 Copper 20-40 88 Copper 79% Tin 21% 28 Silver  5-10 90 Copper 74%Tin 23% Silver  3% 36 Tin 40-89 92 Silver 56% Tin 25% Copper 19%

TABLE 7 (FIG. 9C) Ref. No. Ref. No. from FIG. Thickness from FIG. Atomic9C Composition (μinches) 9C Composition Percent 26 C194 N.A. 26 C194N.A. 32 Nickel 5-20 96 Copper 42% Nickel 32% Tin 26% 34 Copper 7-18 98Copper 50% Tin 41% Nickel  7% Silver  2% 28 Silver 5-10 94 Tin 77%Copper 17% Silver  6% 36 Tin 40-80  92 Silver 56% Tin 31% Copper 13%

TABLE 8 (FIG. 9D) Ref. No. Ref. No. from FIG. Thickness from FIG. Atomic9D Composition (μinches) 9D Composition Percent 26 C194 N.A. 26 C194N.A. 32 Nickel 5-20 100 Tin 41% Nickel 34% Copper 24% Silver  1% 28Silver 5-10 102 Tin 35% Silver 27% Copper 23% Nickel 15% 36 Tin 40-80 92Silver 64% Tin 26% Copper 10%

It is noted that EDX analysis results may vary by a few percent due tox-ray beam spread and depth of penetration. However, for comparisonpurpose, the above results are useful to differentiate among samples.

Although the invention has been shown and described with respect toillustrative embodiments thereof, it should be appreciated that theforegoing and various other changes, omissions and additions in the formand detail thereof may be made without departing from the spirit andscope of the invention as delineated in the claims. All patents andpatent applications mentioned are herein incorporated by reference intheir entirety.

1. An electrically conductive material coated with a plurality oflayers, comprising: a metal or metal alloy substrate; a barrier layerdeposited on said substrate effective to inhibit diffusion ofconstituents of said substrate to said plurality of layers; asacrificial layer deposited on said barrier layer effective to formintermetallic compounds with tin; a low resistivity oxide metal layerdeposited on said sacrificial layer; and an outermost layer of tin or atin-base alloy directly deposited on said low resistivity oxide metallayer.
 2. The electrically conductive material of claim 1 wherein saidbarrier layer is selected from the group consisting of nickel, cobalt,iron, manganese, chromium, molybdenum and alloys thereof.
 3. Theelectrically conductive material of claim 2 wherein said barrier layerhas a thickness of from 4 microinches to 40 microinches.
 4. Theelectrically conductive material of claim 3 wherein said barrier layeris nickel or a nickel-base alloy.
 5. The electrically conductivematerial of claim 4 wherein said nickel or nickel-base alloy barrierlayer has a thickness of from 4 microinches to 20 microinches.
 6. Theelectrically conductive material of claim 4 wherein said sacrificiallayer is copper or a copper-base alloy.
 7. The electrically conductivematerial of claim 6 wherein said copper or copper-base alloy sacrificiallayer has a thickness of from 2 microinches to 60 microinches.
 8. Theelectrically conductive material of claim 7 wherein said copper orcopper-base alloy sacrificial layer has a thickness of from 4microinches to 20 microinches.
 9. The electrically conductive materialof claim 1 wherein said low resistivity oxide metal layer is selectedfrom the group silver, indium, iron, zinc, niobium, rhenium, ruthenium,vanadium, gold, platinum, palladium and alloys thereof.
 10. Theelectrically conductive material of claim 9 wherein said low resistivityoxide metal layer has a thickness of from 5 microinches to 40microinches.
 11. The electrically conductive material of claim 10wherein said low resistivity oxide metal layer is silver or asilver-base alloy.
 12. The electrically conductive material of claim 11wherein said silver or silver-base alloy layer has a thickness of from 5microinches to 20 microinches.
 13. The electrically conductive materialof claim 11 wherein said outermost layer of tin or a tin-base alloy hasan average grain size in excess of 0.5 millimeter.
 14. An electricalconnector assembly including a socket and a probe, with at least one ofsaid socket or probe comprising: a metal or metal alloy substrate; abarrier layer deposited on said substrate effective to inhibit diffusionof constituents of said substrate to said plurality of layers; asacrificial layer deposited on said barrier layer effective to formintermetallic compounds with tin; a low resistivity oxide metal layerdeposited on said sacrificial layer; and an outermost layer of tin or atin-base alloy directly deposited on said low resistivity oxide metallayer.
 15. The electrical connector assembly of claim 14 wherein saidbarrier layer is selected from the group consisting of nickel, cobalt,iron, manganese, chromium, molybdenum and alloys thereof.
 16. Theelectrical connector assembly of claim 15 wherein said barrier layer hasa thickness of from 4 microinches to 40 microinches.
 17. The electricalconnector assembly of claim 16 wherein said barrier layer is nickel or anickel-base alloy.
 18. The electrical connector assembly of claim 17wherein said sacrificial layer is copper or a copper-base alloy.
 19. Theelectrical connector assembly of claim 18 wherein said copper orcopper-base alloy sacrificial layer has a thickness of from 2microinches to 60 microinches.
 20. The electrical connector assembly ofclaim 19 wherein said low resistivity oxide metal layer is selected fromthe group silver, indium, iron, zinc, niobium, rhenium, ruthenium,vanadium, gold, platinum, palladium and alloys thereof.
 21. Theelectrical connector assembly of claim 20 wherein said low resistivityoxide metal layer has a thickness of from 4 microinches to 40microinches.
 22. The electrical connector assembly of claim 21 whereinsaid low resistivity oxide metal layer is silver or a silver-base alloy.23. The electrical connector assembly of claim 22 wherein said outermostlayer of tin or a tin-base alloy has an average grain size in excess of0.5 millimeter.
 24. The electrically conductive material of claim 1wherein the substrate is copper or copper alloy.
 25. The electricalconnector assembly of claim 14 wherein the substrate is copper or copperalloy.